1. Field of the Invention
The present invention relates to a thermoplastic raw material, such as a polyethylene terephthalate (PET) raw material, which contains finely divided, distributed, inorganic and/or organic particles, and films that have improved winding properties and are therefore more suitable than conventional films as capacitor films.
2. Description of Related Art
Particularly for use in capacitors, films used as dielectric materials have to be increasingly thin and smooth in order to reduce the size of the capacitors or to increase the capacitance yield. In addition to a reduction in size, which can be achieved, for example, by reducing the film thickness, it is of interest to increase the capacitance yield further. This is possible in principle by means of smoother films. However, the processing of increasingly thin, conventionally oriented films increasingly leads to the occurrence of irreversible film defects, such as the formation of creases and stretched areas, in the various processing steps, such as winding, metallization, cutting and capacitor winding.
To enable a film to be processed without creases and stretched areas, it requires on the one hand sufficient slip which prevents blocking of the film during the individual process steps. On the other hand, in addition to good slip, the film should have surface topography which permits the air between the individual film layers of the winding to escape sufficiently rapidly. Proposed technical solutions, such as increasing winding tensions, are only of limited use in the case of ultrathin films since an increase in the winding tension may lead to irreversible film defects, such as stretched areas.
In addition to good film handling, the electrical properties, in particular the dielectric strength and the defect behavior, constitute a further criterion for capacitor films. It is known that, when high particle concentrations or large average particle diameters are used, the number of voids increases due to the poor affinity of the inorganic particles for thermoplastic polymers in the molten state and on orientation with the usual orientation ratios. The presence of such voids has an extreme effect on the mechanical properties, such as tensile strength and elongation at break, and on the dielectric strength of the film. In addition, tears occur to an increasing extent during film production, having an adverse effect on productivity and stability of the film production process.
From these points of view, the haze of films with comparable surface roughnesses is a further criterion. The haze of the films is caused on the one hand by the surface haze, due to the surface roughness, and the internal haze, essentially due to (a) the number of particles, (b) the size of the particles, and (c) the size of the voids present around the particles. Accordingly, with comparable surface roughnesses, films having less haze are advantageous.
It is known (see U.S. Pat. No. 3,980,611) that the film handling can be improved by combining small, medium-sized and large particles, depending on the film thickness. In the patent, this is achieved by a combination of large (2.5-10.0 .mu.m) with medium-sized (1.0-2.5 .mu.m) and small (&lt;1.0 .mu.m) particles, it being necessary to fulfill certain relations of film thickness to particle concentration. The disadvantage of these ultrathin films is the relatively greater roughness or the high surface protuberances which are caused by the medium-sized and large particles which result in a lower capacitance yield per unit volume in the capacitor.
It is furthermore known (see EP-A-0 423 402) that films in the thickness range between 0.1 and 4 .mu.m can be produced by adding inert, organic particles obtained by secondary agglomeration and having a particle diameter of from 0.05 to 5 .mu.m and a primary, spherical particle having a particle diameter from 0.05 to 4 .mu.m.
EP-A-0 622 173 states that handling of an oriented monolayer or multilayer film having a total thickness of .ltoreq.4 .mu.m and a roughness of &lt;30 nm can be ensured if the surface gas flow resistance on at least one film surface is t &lt;a.multidot.d.sup.b [sec] (where a=0-10,000 [sec/.mu.m], b=-3 to 0 and d=total film thickness .ltoreq.4 .mu.m). This can be achieved, according to EP-A-0 622 173, by the combination of a spherical particle (I) with a second particle (II) which has an average particle diameter of 0.05-2.5 .mu.m, the average particle diameter of the particles (II) being greater than that of the particles (I).